1. Field of the Invention
The present invention is directed to a drive device for a push-pull stage of the type having a pulse-width modulator or a compare/capture unit in a microcontroller, or a microcontroller fashioned as a pulse-width modulator which varies the pulse-duty ratio of the pulse-width modulated output signal starting with a specific switching frequency and which, with a chronological delay, drives a push-pull stage having at least one upper power switch and one lower power switch.
2. Description of the Prior Art
Drive devices of the general type described above are employed in inverse rectifiers and frequency converters. Inverse rectifiers serve the purpose of converting a prescribed d.c voltage into a specific a.c. voltage having a defined frequency. Frequency converters convert an a.c voltage having a given voltage value and a given frequency into a different a.c. voltage having a (possibly) different frequency or convert a single-phase a.c. voltage into, for example, a three-phase a.c. voltage. In addition to including power electronics (push-pull half bridge or push-pull full bridge), known inverse rectifiers and frequency converters include a pulse-width modulator, or a microprocessor or microcontroller designed for that purpose. Drive devices for circuits and modules of this type are disclosed in many sources in the technological literature, for example in "Anwendungsbeispiele," (applications example) Siemens AG, Bereich Bauelemente, and "Electronic Industrie 7-1992: Drehstrommotorsteuerung mit dem M 37704" referring to a microcontroller manufactured by Mitsubishi. An article in Elektronik 1/1991 describes the use of a 16-bit motor controller which was specifically developed for controlling three-phase motors and brushless d.c. motors. This microcontroller is extremely expensive and is over-dimensioned for use in controlling an inverse rectifier, for example which employs signals having only one phase.
For an inverse rectifier, the alternating voltage across the load (motor, transducer, etc.) should be as close as possible to the voltage value of the d.c. voltage, or of the intermediate circuit voltage. This means that the pulse-duty ratio should change during a cycle so as to exhibit a 100% modulation for the upper half of the sinusoidal wave, and a 0% modulation for the lower half of the sinusoidal wave, i.e., the frequency converter or inverse rectifier must be operated in a full block mode.
In modern power electronics, power semiconductors are operated in a push-pull output stage configuration above a certain power level. Power semiconductors have a so-called protective time associated therewith, which is the time which must exist to prevent an overlap of brief duration of the power semiconductor operating in the switched mode and their pulse-like loading. Failure to observe the proper protective time can result in the destruction of the power semiconductor. The protective time will be of different lengths for different types of power semiconductors (Bipolar, MOSFET, (BT, MCT etc.) and the operation thereof. Storage and delay times of such power semiconductors are also highly temperature-dependent, i.e., the protective times during which neither of the two power semiconductors in a push-pull stage can be simultaneously driven generally lengthen with increasing temperature. The drive and the separation of potential due to the higher-ranking control also have storage and delay times associated therewith. Such power semiconductors are therefore usually driven with shortened drive pulses. The output pulses of the modulator are delayed with a programmable dead time, so that the power semiconductors do not have any switching overlap. For an optimum exploitation of the power semiconductors, the protective time or the dead time must be as short as possible. The protective time or dead time, however, must not be selected so short as to overload the power semiconductor, even under worst-case conditions.